The field of the invention is felts for use in a papermaking machine, and more particularly, press felts for use in the press section of a papermaking machine.
In the conventional papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium, the somewhat self-supporting paper web is transferred to the press section in the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well. It is this press felt which is the subject of the present invention.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated generally cylindrical rolls to remove still further amounts of water therefrom.
Ideally, press felts should have at least the following properties. First, they should have a top surface that is fine enough to produce a smooth finish and minimize marking of the sheet of paper being produced. Second, they should be open enough to allow water to drain through without significant impedance. Third, they should be resilient enough to quickly recover from repeated high nip pressures over a long period of time. Fourth, they should be tough and strong enough to provide good stability, wear resistance and felt life.
Generally, press felts are assembled in the following manner. If the fabrics are not woven endless, the ends are joined by stitching a seam in a conventional manner. The base fabric is then installed on a needle loom, with the fine layer comprising the outside or top loop, where multilayer base fabrics are employed. Batt fibers are applied to the top side or paper contacting surface, in sufficient quantity and weight to give good bulk and cushion properties. The fibers are anchored to the base fabric assembly by one or more needling operations. Thus, the surface of the press felt (top) which contacts the paper web is a felt, formed as the batting material fibers are needled to the base fabric. Fibers may also be needled to the bottom of the woven base fabric to ensure good anchoring of the fibers on the top side.
Needling the entire structure gives the felt a uniform thickness. Needling also provides a cushioned absorbency to the felt and distributes the pressure uniformly across the width of the felt for efficient water removal. Both uniform thickness and pressure distribution significantly reduce or eliminate vibration of the press rolls of the papermaking machine. Needling is necessary to compress the felt to a given density and resiliency and to entangle the fibers in the base fabric so they do not come loose during the papermaking operation. The batt material may be made up of fibers of any of a number of well known compositions, including natural fibers such as wool, but preferably will be made in whole or in part from synthetic materials such as nylon, dacron, etc. In this connection, it is desirable that these fibers be relatively coarse or of large diameter. They will be selected for their stiffness, or "rigidity"; that is their tendency to resist bending or deformation at fiber cross-over points since this enhances their ability to produce a good papermaking surface.
As noted above, the felt should be strong enough to provide good stability, wear resistance, and fabric life. One of the problems with existing felts is that the batt fibers are soft and do not make a strong fabric. Furthermore, the fabrics tend to shift and/or widen on the machine, making it very difficult to predict how a fabric will perform on the press roll. This is a particular problem with fibers made of nylon, since nylon tends to expand upon exposure to water.
Thus, it is an object of the invention to provide an improved papermaking fabric having greater strength and longer felt life.
It is another object of the invention to provide a papermaker's fabric which remains stable on the papermaking machine.
It is another object of the invention to provide a papermaker's fabric which limits expansion upon exposure to water.
It is yet another object of the invention to provide a papermaker's fabric which does not shift or widen upon installation on the paper machine.